The present invention relates to the methods of precision calibration of magnification of scanning microscopes with the use of a test diffraction grate.
Methods of precision calibration of a magnification of synthetic microscopes with the use of a test diffraction gratings are known. In the existing methods test object is positioned and oriented on a microscope table, and corresponding part of the test objects is scanned, with subsequent processing of the thusly obtained data. It is believed that the existing methods can be further improved.
Accordingly, it is an object of the present invention to provide a method of precision calibration of magnification of a scanning microscopes with the use of a test diffraction grating.
In keeping with these objects and with others which will become apparent hereinafter, one feature of present invention resides, briefly stated, in a method of precision calibration of magnification of scanning microscopes with the use of a test diffraction grating, comprising the steps of positioning and orientation of a test object on a table of microscope so that strips of a test diffraction grate are perpendicular to a direction along which a calibration is performed; scanning of a selected portion of the test object along axes X and Y; measuring values of a signal S versus coordinates X and Y in a plane of scanning and storing of said values S (x, y) in a digital form as a two-dimensional digital array; transforming the two-dimensional array of signals S(x, y) into a two-dimensional array S (u, v) by turning of the axes so that a direction of a new axis u is perpendicular to the strips of the grating and a direction of a new axis v coincides with the strips of the grating; line-by-line mathematical processing of the array S(u,v) for each line S(u) by calculating of a Fourier spectrum of the line SP(xcfx89) in correspondence with the formula             SP      ⁢              (        ω        )              =                  1                              2            -                              ⁢                        ∫                      -            ∞                    ∞                ⁢                                            S              ⁢                              (                u                )                                      ·            exp                    ⁢                      xe2x80x83                    ⁢                      (                          ⅈu              ⁢                              xe2x80x83                            ⁢              ω                        )                    ⁢                      xe2x80x83                    ⁢                      ⅆ            u                                ,
wherein xcfx89 is a coordinate in a reciprocal space which represents a space frequency; SP(xcfx89) is a complex spectrum density which corresponds to the space frequency xcfx89; S(u) is a function which describes a one-dimensional profile of the signal; and i={square root over ( )}xe2x88x921 is an imaginary unit; converting of the one dimensional complex function SP (xcfx89) into a one-dimensional spectrum of real values of a module |SP(xcfx89)| by multiplying of each value SP(xcfx89) by a complex-conjugate value; finding from the spectrum of the real values T the greatest spectral maximum and determining its characteristic frequency xcfx89h as an abscissa of a point with the maximum value |SP|; calculating an average value of a pitch Th of the diffraction grate in accordance with the formula:       T    h    =      1          W      h      
performing the mathematical processing, the conversion, the determination and the calculation for subsequent lines S(u) with a new value of a coordinate v; statistically processing the thusly obtained set of values Th for all lines and determining an average value T and a standard deviation xcex94T over a whole frame; and determining a magnification Mu in accordance with the selected direction u in accordance with the formula:             M      u        =                  T        ·        L                              T          0                ·        N              ,
where L is a width of a medium of image in direction of the calibration, T0 is an independently obtained value of the pitch of the same test object, and N is a number of pixels in the line along the direction u.
When the method is performed in accordance with the present invention, it reliably provides a precision calibration of magnification of scanning microscopes, and achieves a very high accuracy of calibration.